Submerged arc crystal growth process for growing transparent alkaline earth oxide single crystals

ABSTRACT

AN IMPROVED METHOD FOR GROWING TRANSPARENT SINGLE CRYSTALS OF ALKALINE EARTH OXIDES BY THE SUBMERGED ARC METHOD WHEREIN AFTER FUSION OF A CHARGE THE POWER PLIED TO THE ARC IS REDUCED TO 20 TO 40 PERCENT OF THE ORIGINAL VALUE AND MAINTAINED AT THE REDUCED VALUE FOR THE DURATION OF THE RUN.

United States Patent 3,829,391 SUBMERGED ARC CRYSTAL GROWTH PROCESS FOR GROWING TRANSPARENT ALKALINE EARTH OXIDE SINGLE CRYSTALS Yok Chen and Marvin M. Abraham, Oak Ridge, Tenn.,

assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Sept. 19, 1972, Ser. No. 290,296 Int. Cl. Ctllf 11/06 US. Cl. 252-301.4 R Claims ABSTRACT OF THE DISCLOSURE An improved method for growing transparent single crystals of alkaline earth oxides by the submerged arc method wherein after fusion of a charge the power supplied to the arc is reduced to 20 to 40 percent of the original value and maintained at the reduced value for the duration of the run.

BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission. It relates generally to the art of crystal growth by the submerged arc technique.

It is well known that various refractory oxide single crystals may be grown by the submerged arc technique. According to this technique an appropriate powder is placed in a water-cooled container and at least two, but preferably three, electrodes are submerged within the powder. When an arc is struck between the electrodes, the powder surrounding the arc is fused by the heat generated from the arc. The amount of heat supplied is controlled by both power regulation and by electrode separation. The fusion of the powder surrounding the arc causes a cavity to form within the volume immediately surrounding the arc. Below the cavity is a layer of fused material which rests on a stratum of sintered material. During the process the border between the sintered stratum and the fused layer moves downward because of the crystallization of the fused layer upon the sintered stratum. The crystals, which are eventually harvested, come from the fused layer. The radius of the cavity and, therefore, the amount of crystals at the bottom of the cavity is determined in a large part by the power generated at the arc. After a suflicient length of time at a high power level, the power supplied to the electrodes is usually shut off to allow for cooling. After cooling, a boule containing the crystals is removed from the surrounding powder. The individual single crystals are then separated from one another. The process is very attractive for growing crystals of high purity since there is no foreign crucible material to contaminate the crystals.

A problem encountered in the growth of crystals by this technique is that a large number of small bubbles of the order of a micron in diameter are present in the resulting crystals. These bubbles cause light scattering which reduces the transparency and renders the crystals translucent. This problem is much more pronounced when using powder of high purity, i.e., greater than 4 N (99.99% pure). Single crystals which are completely transparent are required for applications such as lasers and photovoltaic devices. Prior art attempts to solve this problem have not been completely successful. One such attempt involved the use of an inert cover gas during the process. The cover gas is provided by using hollow carbon electrodes and passing an inert gas through the hollow portion, so as to sweep the melt with the gas. This technique produces crystals with a smaller amount of larger bubbles than was produced by use of the prior technique. However, even though the light scattering is somewhat reduced crystals produced by the cover gas technique do 3,829,391 Patented Aug. 13, 1974 not meet the standards for present technological applicatious.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide a method of growing crystals by the submerged arc technique which produces large totally transparent single crystals.

The above object is accomplished according to this invention by first forming a boule from a starting powder in the usual manner and then reducing the power supplied to the arcs to a level which is about 20 to 40 percent of the initial level and maintaining the reduced level for the duration of the process.

DETAILED DESCRIPTION According to this invention it has been found that high purity bubble-free single crystals may be grown by the submerged arc technique. It is, of course, understood that the process of this invention is not limited to high purity materials but can also be used to grow crystals of low purity materials as well as doped materials. However, the beneficial eifect of the process of this invention is more pronounced with high purity materials since inherently these materials appear to develop more bubbles than low purity materials.

Any oxide material which is capable of crystal growth by the submerged arc technique can be used in the process of this invention. The alkaline earth oxides have proved to be particularly applicable to the process of this invention. The hygroscopicity of the alkaline earth oxides has always been a problem in producing bubble-free crystals, since water is unavoidably present in the starting powders. In some cases a carbonate of an alkaline earth oxide is used as a starting material. The release of CO upon heating the carbonate provides another source of gas for inclusion into the resulting crystals. In the cases of CaO and SrO, CaCO and SrCO respectively, have been used as starting materials. The carbonates decompose during the initial heat to CO and the oxide.

In growing crystals by the submerged arc technique the wattage requirement is dependent upon the geometry of the overall apparatus. If a three-phase power source and a three-electrode system are used, the initial power is about 40 to 50 kw. for up to a 60-kilogram load in a typical run of which about 25 percent in weight will be fused and about 15 percent of the total weight will result in crystals. A three-electrode system is preferred for use with the process of this invention because of the larger volume of single crystals obtained thereby.

When beginning a crystal growth run it is preferred to use a powder which has been isostatically compacted or sintered material from a prerun. The electrodes are then placed through the powder to a point of near contact. A bridging electrode is placed through the top of the powder so as to establish contact with the other electrodes. At this point current flow is begun. When the temperature reaches a level to enable sustenance of the are (approximately 5- 10 minutes) the bridging electrode is removed and the arc is established between the other electrodes. The opening through the powder which housed the bridging electrode now serves as an escape port for gases released during heating. The three electrodes are moved away from the initial point of contact a predetermined distance which defines the volume in which fusion and crystal growth will occur.

The power requirements for forming the initial boule as described above are readily determined by one of ordinary skill in the crystal growth art. The power requirements will, of course, vary for different refractory oxide systems. Using a satisfactory power setting, it is preferred, according to this invention, to maintain this power level for about 40 minutes to 2 hours in order to obtain large single crystals.

At this point in the submerged arc technique of crystal growth, the process of this invention differs greatly from that used in the prior art. It has been found that if the power supplied to the electrodes is reduced to a value which is 20 to 40 percent of the value required for crystal growth and maintained at the reduced value for a period of time suflicient to allow included bubbles to migrate to the crystal surface, transparent crystals are obtained. This generally requires about 6 to 14 hours. While the surface of the individual crystals in the boule becomes very cloudy, the bulk appears to be completely transparent. Occasionally, however, a single large bubble, of the order of 1 mm. in diameter, can be found in the bulk material. The surface of these crystals can easily be polished, etched or cleaved ofi to yield a transparent crystal.

It is preferred, according to this invention, to reduce the power level as quickly as possible in order to conserve the electrodes for use at the lower power level. Preferably, the power level is reduced within a time period of less than 15 minutes. However, longer periods of time may be used if the electrodes are of suflicient length to insure a long period of operation at the lower point level.

Although the improvement in crystal clarity produced by the process of this invention is surprisingly superior to expectations based upon just a long anneal period, the following non-limiting theory is presented as a possible explanation of the observed phenomenon.

Immediately following crystal growth at high power, the crystals have many small microbubbles just as the prior art crystals do. The microbubbles are extremely high pressure gas trapped within the confinement of the crystals. By maintaining the crystals at high temperature for a sufficiently long period, the microbubbles have ample opportunity to diffuse and eventually work their way to the surface. The power level for this procedure is quite critical. If the power level is too low, the temperature may not be sufficiently high to ensure sufficient mobility of the microbubbles; furthermore the arc may not sustain itself. On the other hand, if the power is too high, the crystals will not be completely transparent, no matter how long the arc is sustained.

In order to better illustrate the process of this invention, the following specific example is given.

A crystal growth apparatus comprising a water-cooled stainless steel tub with a hollow depression in the form of an inverted tetrahedron with chamfered edges and a truncated bottom, capable of holding about 60 kg. of MgO powder, was used in carrying out the process. Three carbon electrodes are moveably mounted parallel to the three chamfered edges. The three electrodes are at an angle of 42 with the vertical and meet at a point in the center of the depression about 25 centimeters above the truncated bottom.

Power for the system is supplied from 440 v. mains, stepped down to 150 v., and controlled manually by a three phase 80 kva. saturable core reactor (Sorgel Electric Corporation, Milwaukee, Wis.). The graphite used for the electrodes is grade UF4-S (Ultra-Carbon Corporation, Bay City, Mich.).

About 30 kg. of SrCO powder was placed in the depression of the apparatus. The three carbon electrodes were immersed in the powder to a point of near contact, and a fourth bridging electrode was vertically positioned through the top of the powder so as to establish contact with the other three electrodes. Current flow was begun at a power of l() kw. After eight minutes the bridging electrode was removed, thus establishing an arc between the three power electrodes. After about a minute the three electrodes were pulled out 3.5 centimeters from the original point of near contact. The power level was about 34 kw. After another minute the electrodes were pulled out a centimeter and the power level rose to 42 kw. As the powder heated, the power level dropped over the next three minutes to about 36 kw., at which point the electrodes were pulled out an additional 0.5 centimeter causing the power level to rise to 40 kw. The power level again dropped over the next three minutes to 38 kw. The electrodes were pulled out another one-half centimeter causing the power level to rise to 41 kw. After six minutes the power level had dropped-to 37 kw. and the electrodes were pulled out one-half centimeter causing the power level to rise to 40 kw. After eleven minutes the power level was 36 kw., at which point the electrodes were pulled out an additional one-half centimeter causing the power level to rise to 39 kw. After a total of 40 minutes, the tip of the electrodes was at a distance of more than the 6.5 centimeters withdrawn from the original starting point due to oxidation of the electrodes, thus placing them at the corners of an equilateral triangle about 12-15 centimeters from one another. At this point the power level was cut to 14 kw. by both turning down the power level and by pushing the electrodes into a position of about two centimeters from the center starting position. After seven minutes at the reduced power level, the power level had risen to 16 kw. and the electrodes were pushed in one centimeter to reduce the power level to 14 kw. After an hour the power level had risen to 16 kw. where it remained for the next four hours. The electrodes were then pushed in about one centimeter to obtain a power level of 14 kw. The electrodes were periodically pushed in to maintain a power level of about 14 kw. After a total of eight hours and fifteen minutes at the reduced power level, the power was turned off. The material in the apparatus was allowed to cool overnight before removal. Beneath the arc was found a conglomerate of single crystals, most of which were about 10 cm. in size, and some as large as 25 cm. Immediately below the single crystals was the sintered strata of SrO. The surfaces of the single crystals were mostly cloudy, but the bulk was totally transparent.

Both CaO and MgO single crystals have been grown by the above technique. The apparatus holds about 60 kg. of CaO or MgO and requires an initial power setting of about 45 kw. for CaO and about 50 kw. for MgO. After about 40 minutes to 2 hours the power setting is reduced to 10 to 15 kw. for CaO and MgO and retained at the reduced level for about 6 to 14 hours to obtain transparent crystals.

The process of this invention has also been used for growing transparent doped crystals. The dopant oxide powder is merely mixed with the host powder prior to placing it in the apparatus. MgO has been doped with deuterium, Li, Na, K, Ag, Ce, Eu, Gd, Dy, and Yb. CaO has been doped with Li, Na, K, Ag, and Nd. SrO has been doped with Li, Na, K, Ce, Ag, Gd, and Yb.

Thus, it is apparent that the process according to this invention can be used for growing transparent crystals in systems which heretofore could not be obtained as totally transparent single crystals.

What is claimed is:

1. In a method of growing alkaline earth oxide single crystals of alkaline earth oxides by the submerged arc technique wherein a plurality of electrodes are placed in a mass of powder, said powder being either an alkaline earth oxide or an alkaline earth carbonate, and a first electrical power level established between said electrodes such that an arc is generated between said electrodes sufficient to fuse said powder and cause crystal growth to occur, the improvement comprising the step of reducing the power after said powder is fused to a second power level which is 20 to 40 percent of said first electrical power level and maintaining said second power level until substantially transparent single crystals are obtained.

2. The method according to claim 1 wherein said powder as an alkaline earth oxide.

3. The method according to claim 1 wherein said powder is a carbonate and decomposes to form said oxide.

4. The method according to claim 1 wherein said step of maintaining comprises a time period of about 6 to 14 hours.

5. The method according to claim 1 wherein said first power level is maintained for a time period of about 40 minutes to 2 hours.

6. The method according to claim 1 wherein said step of reducing comprises a time period of less than about 15 minutes.

7. The method according to claim 1 wherein said first power level of 40 to 50 kilowatts and said second power level is about 8 to 20 kilowatts.

8. The method according to claim 1 wherein said oxide is selected from the group consisting of MgO, CaO and SrO.

9. The method according to claim 8 wherein said powder is of 99.99 percent or greater purity.

10. The method according to claim 1 further comprising mixing a dopant oxide with said powder whereby doped oxide crystals are produced and said dopant oxide is an oxide of a metal selected from the group consisting of lithium, sodium, potassium, silver, cerium, europium, gadolinium, dysprosium, ytterbium, and neodymium.

References Cited UNITED STATES PATENTS OTHER REFERENCES Kovalev et al.: Chemical Abstracts, vol. 75, 1971, 102067t.

JACK COOPER, Primary Examiner.

U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,829,391 Y Dated August 13, 1974 Inventor(s) YOK Chen et a] It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, hne v22, "point" shou1d read -power--.

Column 4, line 56, de1ete "of a1ka11'ne earth oxides".

C01 umn 4, line 69, "as" shou1d read 1's-.-.

Column 5, line 8, "of" shou1d read --1's-.

Signed and sealed this 17th day of December 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Qgmmissioner of Patents FORM PC4050 a) USCOMM-DC scan-P69 U.S. GOVERNMENT PRINTING OFFICE: I969 0-365-334 

